A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102, which moves the bones of the middle ear 103 that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by a central cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. When the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processor 111 in which various signal processing schemes can be implemented. The processed signal is then converted into a digital data format for transmission by external transmitter coil 107 into the implant device 108. Besides receiving the processed audio information, the implant device 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces an electrical stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. The electrode array 110 includes multiple electrode contacts 112 on its outer surface that provide selective stimulation of the adjacent neural tissues within the cochlea 104.
The implant device 108 connects to the electrode lead 109 at an electric feedthrough on the outer surface of the implant device 108. The electric feedthrough includes an electrical insulator which contains one or more electrically conductive connectors. The feedthrough is surrounded by an outer ferrule that is hermetically sealed to the housing of the implant device 108. In addition, the electrical insulator and the conductive connectors also must form a hermetical seal within the ferrule to prevent fluids and moisture from entering into the interior of the implant device 108.
To form the required hermetic seal of the electrical feedthrough, the critical issue is the interface between metallic feedthrough ferrule and the ceramic material of the implant device. Ceramics and metals have different material properties which make it difficult to seal the dissimilar materials. Ceramics exhibit an ionic bonding while metals have a metallic bonding that complicates wetting of the ceramic with the metal, which results in poor adhesion of the metal to a ceramic surface. The ceramic and the metal materials also exhibit different thermal expansion coefficients which leads to thermal stress problems.
Various techniques are available to hermetically seal glass/ceramics and metals. For a hermetic glass-to-metal seal, the molten glass must be capable of wetting the metal so that a tight bond is formed. The glass and the metal are strongly bond together when the oxide layer at the metal surface chemically interacts with the glass. In addition, the thermal expansion coefficients of the glass and the metal need to match in order to achieve a stable seal when the assembly cools down. Further hermetic connections can be provided by glass-ceramic-to-metal seals. Glass-ceramics are polycrystalline ceramic materials formed by controlled crystallization in order to adapt the thermal expansion coefficient of the glass-ceramics to the one of metal. Glass-ceramic-to metal seals are in use for electrical feedthroughs in vacuum application or for hermetic and insulating sealants in solid oxide fuel cells.